The present invention relates to the realization of microwave bandpass filters with dual mode resonance cavities arranged so as to achieve asymmetrical transmission characteristics.
Microwave bandpass filters are widely used in terrestrial and space telecommunications systems in order to provide noise or interference rejection and in multiplexers where they are used for low loss combination or separation of different transmission channels. The majority of these characteristics are symmetric and have been realized in microwave structures that are synchronously tuned, i.e. structures in which all resonators are tuned to the same center frequency.
In some particular applications, however, it is desirable to provide asymmetric transmission characteristics. A first application, for instance is the outer channel filters in a contiguous-channel multiplexer where the absence of a neighbour channel on one side causes a severe asymmetric distortion of the in-band group delay and insertion loss characteristics. This asymmetric distortion can be very damaging to digital signals and, if uncorrected, will require higher transmitter powers to restore the bit error rate to that of the undistorted case. Another major application is within transmission systems which have asymmetric rejection specifications, for example in a receive channel with an adjacent transmit channel which has to be heavily rejected.
Considering the physical implementation an interesting construction uses in-line dual mode resonance cavities. FIG. 1 shows schematically an exploded view of a two-cavity implementation. The two cylindrical cavities 100 and 200 are separated by a plate 300 having a cruciform coupling iris 400 therein. Each cavilty supports two TE.sub.11 mode resonances, polarized orthogonally to each other and tuned individually by means of a tuning screw. These two resonances are coupled by means of a coupling screw located at 45.degree. to the tuning screws. Coupling between resonances in adjacent cavities is achieved with the cruciform coupling iris 400. This type of construction only realizes transmission characteristics which are symmetric about the center frequency because the starting point is always a folded prototype network which is essentially symmetric (FIG. 2). This is a folded ladder network allowing cross-coupling between non adjacent shunt capacitors. These cross-couplings are designated by the symbols K18, K27, K38. Such a network is in effect the electrical embodiment of the characteristics which are defined in purely mathematical form by transfer polynomials. The process for converting these transfer polynomials to the folded electrical network has been described by J. D. Rhodes in: "A Low-Pass Prototype Network For Microwave Linear Phase Filters (IEEE-MTT, Vol. MTT-18, June 1970, pp. 145-160).
In order to create asymmetric responses it is required to construct a structure corresponding to an electrical prototype network comprising diagonal couplings as shown at K17, K26, K35 in FIG. 3.